Water and Environment Management Framework Lot 3 – Engineering and Related Services Slowing the Flow in the Rivers Ouse & Foss July 2017 Document overview Capita AECOM was commissioned by the Environment Agency in May 2016 to undertake the York Flood Management which has comprised this review of the potential for upstream storage and Natural Flood Management to provide opportunities to manage the flood risk faced by the City of York. Document history Version Status Issue date Prepared by Reviewed by Approved by 0 Draft March 2017 A Gee G Knott C Lomax 1 Proof Copy April 2017 A Gee G Knott C Lomax 2 Final July 2017 A Gee G Knott C Lomax AECOM Infrastructure & Environment UK Ltd 5th Floor 2 City Walk Leeds LS11 9AR UK T: +44 (0)113 391 6800 aecom.com i Limitations This Report has been prepared for the sole use of the Environment Agency in accordance with the Agreement under which our services were performed. 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Copyright © This Report is the copyright of AECOM Infrastructure & Environment UK Ltd. Any unauthorised reproduction or usage by any person other than the addressee is strictly prohibited. ii Contents 1. Introduction 1 2. The Upstream Catchments & Flood Risk in York Today 6 3. The 5-Year Plan 13 4. Catchment Scale Measures to Reduce Flood Risk in York 18 5. Upstream Storage 22 6. Natural Flood Management 33 7. Engineering Measures 44 8. Development and rural land use planning 48 9. Flood Forecasting & Warning 51 10. Total Catchment Management 56 11. Summary 59 Figures Figure 1.1: Response to the Boxing Day 2015 flood event 1 Figure 1.2: The catchments upstream of York 3 Figure 1.3: Flooding on the 28th December 2015 looking upstream from the confluence of the Rivers Ouse and Foss 4 Figure 1.4: Flooding on the 28th December 2015 on the River Foss looking west 5 Figure 2.1: The steady increase in the annual maximum levels at the ‘Viking’ river level recorder in central York 10 Figure 3.1: Existing flood defences 15 Figure 4.1: Location of river gauging stations reviewed 19 Figure 4.2: Tributary hydrographs resulting in the Boxing Day 2015 flood event 20 Figure 5.1: Location of existing reservoirs upstream of York and existing washlands 25 Figure 5.2: Myton Ings (River Swale) during November 2000 event, looking upstream 27 Figure 5.3: Structures restricting the Foss flow downstream 29 Figure 5.4: River Ouse, November 2000 looking upstream of York 32 Figure 6.1: The impact of land management on flood risk. NFM seeks to reinstate / emulate the catchment on the right 33 Figure 10.1: Flood risk management activities 56 Figure 10.2: Total Catchment Management 58 Figure A3: Catchment scale classification of NFM measures, coloured arrow indicates increasing understanding and reliability (adapted from Thorne et al., 2007) 65 Figure A4: Catchments feeding the River Ouse through York 72 Figure B1.1: River Foss NFM Study area in relation to surrounding catchments and the City of York 92 Figure B1.2: Overview of Foss NFM Study methodology 93 Figure B2.1: River Foss, tributaries and key catchment features 94 Figure B2.2: LiDAR coverage in the Foss catchment 95 iii Figure B3.1. Reach Identification and Numbering 99 Figure B3.2. Numbering of sub-reaches within the Foss catchment. 100 Figure B3.3. Modelled Extents. 103 Figure B3.4. Flood Modeller routing model of the Foss Catchment. 106 Figure B6.1. Location at which modelled results were taken. 116 Figure B6.2. Impact of Scenario 5 interventions within the River Foss sub- catchment for the 1% AEP event 117 Figure B6.3. Impact of Scenario 5 interventions within the Tang Hall Beck sub-catchment for the 1% AEP event 118 Figure B6.4. Impact of Scenario 5 interventions within the Osbaldwick Beck sub-catchment for the 1% AEP event 119 Tables Table 4.1: River gauge data 19 Table 5.1: Upstream storage volumes required for level reductions at York 22 Table 5.2: River Foss Storage Volumes 23 Table 5.3: Existing Upstream Storage Reservoirs 23 Table 5.4: Existing washlands 27 Table A5: River and catchment based NFM measures (adapted from Natural Flood Management Handbook, SEPA, 2015) 64 Table A6: Barriers to implementation of NFM measures 71 Table A7: Comparison of the SUNO, Foss and Pickering catchments during the Boxing Day floods (December 2015). 74 Table B2.1: Water Framework Directive Status of waterbodies within the Foss catchment 97 Table B3.1: Reach typology classification (after Chow, 1959) 101 Table B4.1: List of NFM measures for identification within the catchment 108 Table B4.2: Opportunities and constraints for Woody Debris 109 Table B5.1. Adjustments to Manning’s ‘n’ to represent NFM measures within the Foss catchment (after Arcement & Schneider, 1984) 112 Table B6.1. Impact of Scenario 5 interventions at downstream end of modelled watercourses. 119 Table B6.2. Impact of intervention of peak flow at downstream end of modelled watercourses when compared to baseline, broken down by scenario. 120 Table B6.3. Impact of intervention on timing of peak flow at downstream end of modelled watercourses when compared to baseline, broken down by scenario. 121 Appendices Appendix ANatural Flood Management Literature Review Appendix BNatural Flood Management Technical Report iv 1. Introduction Storms Eva and Frank in December 2015 brought record rainfall across Yorkshire with over 600 households and businesses flooding in York on the 26th December 2015. The flood event was amongst the most significant in Yorkshire since current records began: many locations recorded their highest ever flood levels. The river level on the River Ouse in York reached its second highest peak in the recent record, whilst the River Foss recorded its highest ever river flow. Since December 2015, the Environment Agency, City of York Council and partners have taken action to reduce the risk of flooding in York. This has included: x Immediate recovery: the immediate response during the event, and in the aftermath of the flooding, including increasing the pumping capacity of the Foss Barrier; x Short term: preparing a 5-year Plan to manage flooding in York in the short term through flood alleviation measures and providing consistent standards of protection within the City of York area; and x Long term (this document): a review of means by which to manage the risk of flooding in York over the next 100 years, through flood alleviation measures upstream of York. These are shown graphically in Figure 1.1. Figure 1.1: Response to the Boxing Day 2015 flood event Chapter 2 of this document describes the catchments of the Rivers Ouse and Foss which flow through York. It discusses the flood risk these rivers pose, documents the recent historical flood history and introduces how river flows are changing over time. The chapter also identifies other sources of flooding in York. Chapter 3 presents a high level summary of the existing flood defence assets in York and the Environment Agency’s 5-year Plan to manage flooding in York. The 5-year Plan comprises possible interventions through the City of York to provide additional measures to alleviate the effects of flooding. This document has been designed to supplement the 5-year Plan, to consider holistic solutions upstream of the city to alleviate flooding in York for the next 100 years. 1 Chapter 4 introduces the catchment scale measures that have been considered as part of this document to slow the flow that reaches York during a flood event, which are covered in the subsequent chapters and discusses the hydrology of the upstream catchment identifying characteristics requiring further consideration if catchment-scale measure are to be implemented upstream of York: x Chapter 5 looks at providing washlands to store water upstream of York during a flood event; x Chapter 6 reviews the potential of Natural Flood Management (and sustainable drainage in urban areas) to provide benefits to York; x Chapter 7 provides a high level consideration water level control and other engineering options that could be implemented outside of the city but would benefit the city; x Chapter 8 touches on the role of development and rural land use planning; and x Chapter 9 discusses the role of flood forecasting and warning.